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This research examined genetic, morphological, ecological and behavioral mechanisms
maintaining divergence between a pair of sympatric stickleback species (Gasterosteus aculeatus
complex) from Paxton Lake, British Columbia, Canada.
When raised in the lab, F₁ hybrids showed no evidence of inferiority relative to parental species
for the following characters: fertilization success, hatch success, growth rate, fecundity, fluctuating
asymmetry, and parental care. Although these are common postmating isolation mechanisms between
many other species, if they operate in the wild in this system their effects are likely small and of little
biological importance. When raised in the wild, in either of two main habitats, F₁ hybrids suffered a
significant reduction in growth rate relative to parental species. Hybrid disadvantage resulted from
morphological intermediacy which affected resource exploitation efficiency. Hybrid disadvantage is thus
a function of the ecological environment, rather than developmental inviability or physiological
inferiority. This ecological disadvantage provides the strongest postmating barrier known between these
species. Though this is one of the first demonstrations that niche-based selection pressures are a dominant
mechanism of postmating isolation, I argue that this may be common in the early stages of speciation.
Since gene flow between species must occur through reproductively fit F₁ hybrids, I asked
whether male F₁ hybrids are sexually selected against by females of the parental species. I provide
experimental evidence that mate choice by females of the parental species must be coupled with habitat
preference for there to be sexual selection against hybrids.
I measured the degree of genetic divergence between the species in four morphological characters
and one fitness component by rearing six lines (parentals and first and second generation hybrids; 109
families total) in the lab. Joint-scaling tests on phenotype means and variances suggest that epistasis
contributes significantly to genetic divergence of parental lines for two of the characters, gill raker length
and growth rate. Dominance effects contribute significantly to divergence in plate number and pelvic
spine length. A simple additive model was sufficient for only gill raker number. A biometrical approach
to gene number estimation suggested that character differences are coded for by 1 to 60 genes, depending
on the trait. The tremendous discrepancy in these estimates may reflect real differences among characters
in genetic divergence, but the estimates are likely unreliable due to the rejection of several important
assumptions of the method. My findings are compared to other recent estimates of genetic divergence in
natural populations, and discussed in the context of speciation in sticklebacks.